BLUETONGUE
Bluetongue is a non-contagious viral disease with high morbidity and mortality rates in ruminants. The disease is spread very easily through Culicoides midge bites and has a high propagation potential. Therefore, bluetongue is in the World Organization for Animal Health (WOAH) Terrestrial Animal Health Code and a European Commission listed disease.
Cattle
Sheep
Goats
Camelids
It can also affect various wild species like buffalo, deer, elk, gnu and antelope.
Bluetongue virus (BTV) is endemic in tropical and subtropical regions of Africa, Asia, Australia, Europe and North America, as well as various tropical and subtropical islands. However, the geographic distribution and behavior of BTV have changed significantly in recent years. These changes are mainly attributed to increased international livestock trade and climate change, which has allowed the midge vector to survive in regions that were previously too cold.
Since 2006, bluetongue has spread considerably in Western Europe, seen as far north as 58°N and putting at risk many areas previously believed to be out of harm’s way.
- High temperatures: As temperatures rise, so does midge activity.
- Standing water: Areas with standing water are a perfect place for midges to breed.
Asymptomatic infected animals can pass the virus to midges, which once infected are carriers for life and can infect other ruminants.
Animals in areas where the virus is endemic tend to develop a certain natural resistance to BTV. However, the risk increases when susceptible breeds, like those from Europe, are introduced in endemic regions or when the virus spreads to new areas where it wasn’t previously present.
The bluetongue virus belongs to the serogroup of epizootic hemorrhagic diseases of the Orbivirus genus and reoviridae family. Currently, 27 BTV serotypes have been identified.
The main vector that spreads the virus is the Culicoides midge. It is highly unlikely for the virus to spread directly between animals. In periods of high temperatures and abundant rainfall, the number of cases increases along with the increase in vector activity. The spread tends to drop with the first freeze or intense cold.
Cattle are the main reservoir of the virus in most regions. These animals often carry the virus without any clinical symptoms and can have viral load in their blood for a very long time, up to 100 days. This contributes to the silent spread of the virus, particularly in areas where host animals have developed resistance to the disease.
The bluetongue virus is transmitted when a healthy midge bites an infected animal. The virus is ingested with the blood, infecting the midge’s intestinal cells then replicating and migrating into the hemocoel (the insect’s body cavity). Once the virus reaches its saliva glands, the midge can spread it to a new host through its bites.
There are two main ways BTV is spread:
Biological vector
Infected midges are the main means of transmission. Normally, these insects fly short distances, between 1 and 2 kilometers, but can sometimes be carried on the wind for more than 300 kilometers, which helps the virus spread quickly.
This type of transmission only occurs with specific serotypes, such as serotype 8. In this case, the virus is spread from the mother to her offspring before birth.
It is important to note that BTV is not spread to humans through contact with animals, wool or milk.
The bluetongue virus incubation period is generally between 4 and 8 days. Clinical cases of the disease vary depending on the species affected, with sheep being the most prone to showing significant symptoms.
In cattle, subclinically infected animals can be contagious between 2 and 4 days after infection. The onset of clinical signs in cattle depends on the virus serotype they are infected with.
The most common clinical signs in cattle include:
- Skin lesions: like vesicular and ulcerative dermatitis, periocular dermatitis, necrotic wounds and photodermatitis.
- Reproductive losses: including abortion, stillbirths and congenital abnormalities.
- Congenital lesions: in calves infected early in gestation, including blindness and central nervous system abnormalities like hydranencephaly and microphthalmia.
- Decreased milk yield.
Although mortality is rare in adult cattle, complications in calves can be serious and cause death soon after birth.
In sheep, morbidity can reach 100% and the recovery process is long and can have side effects like wool loss, sterility and delayed growth. The mortality rate in sheep is roughly 30%, although for certain more susceptible breeds it can be up to 70%.
The most common clinical signs in sheep include:
- Fever up to 42°C
- Excessive salivation, mucopurulent nasal secretion and panting
- Depression and weakness
- Ulceration and necrosis of the mouth (blue tongue)
- Hyperemia and swelling of the tongue that can spread to the hooves, groin, axilla and perineum
- Wounds on legs and hooves and limping
- Temporary infertility, spontaneous abortion or deformed fetuses
- Weight loss and malnutrition
- Acute diarrhea and vomiting
- Pneumonia
- Break in wool growth
In other domesticated ruminants, like goats, the disease is usually subclinical, although they can be carriers of the virus and help it spread.
Mortality in wild ruminants like deer and antelopes can be significantly higher, reaching 90%.
This poses a significant risk to biodiversity and can have ecological and economic repercussions.
High mortality
Bluetongue can have a mortality rate of up to 70% in an infected flock.
Abortions in infected animals causes the birth rate to drop up to 80%.
On numerous occasions, in order to prevent or slow the spread of the virus in certain areas, some countries have imposed restrictions on cattle trade.
The animals’ poor physical health leads to decreased yield of milk and meat, and wool in sheep.
The global economic impact of bluetongue disease is estimated to be roughly €2.8 billion ($3 billion).
In 2007, a study was done to analyze the financial impact in France and the Netherlands, which found losses of €1.3 billion ($1.4 billion) and €80 million ($85 million) respectively. In Germany, the economic impact of the epidemic from the first cases in 2006 through 2018 has been estimated at €157 million to €203 million ($167 million – $216 million).
Most of these losses are categorized as direct production losses due to mortality, abortions, lower fertility rate, congenital abnormalities and lower meat yield. Indirect losses include vaccination costs, lost income due to commercial restrictions, and having to cull animals when the disease can’t otherwise be controlled.
How to control and prevent bluetongue disease?
There is no specific treatment for animals infected with the bluetongue virus beyond rest, soft foods and good rearing. So, prevention is key to fighting the spread of BTV.
In areas where the disease is endemic, monitoring programs are required to detect the virus and monitor its progression. In European Union member states, there are a series of strategies that include:
In combination with active vigilance programs to identify the location, distribution and prevalence of vector insects in the area, it can also include appropriate control measures like:
Quarantine and/or restricted movement during insect activity periods: To control and limit potential epidemics.
A zone can only be declared free of BTV when there is an ongoing vigilance program that has not found Culicoides midges or animals infected with the virus for at least two years.
In vitro and in vivo cultures
In a lab, the bluetongue virus is isolated in a sample or product from a ruminant or camelid.
Serological diagnosis
Detect BTV antibodies. Serological response appears 7-14 days after infection. The sample must be from an unvaccinated animal because vaccinated animals have antibodies despite never having been infected.
Quickly identify the BTV viral nucleic acid in the blood or other tissues of infected animals. It is important to note that this test detects nucleic acid once the virus is no longer viable and, therefore, a positive result doesn’t necessarily indicate the presence of infectious virus. Vaccinated animals can also give false positives.
Vaccination is used as the most effective and practical way to control the bluetongue virus in order to limit associated financial losses, interrupt the cycle of infection and allow animals to move about safely. It is essential to use the right vaccine designed to provide protection against the strain (or strains) that are most prevalent in the country or region.
Inactivated bluetongue virus vaccines are very safe, as there is no risk of contracting the disease from them. This type of vaccines has been very effective in fighting the spread of BTV in Europe.
- Desmecht, D., Bergh, R., Sartelet, A., Leclerc, M., Mignot, C., Misse, F., Sudraud, C., Berthemin, S., Jolly, S., Mousset, B., Linden, A., Coignoul, F. and Cassart, D., (2008). Evidence for transplacental transmission of the current wild-type strain of bluetongue virus serotype 8 in cattle. The Veterinary record, 163(2), 50-52.
- Gethmann, J., Probst, C. & Conraths, F.J.(2020). Economic Impact of a Bluetongue Serotype 8 Epidemic in Germany. Frontiers in Veterinary Science vol.7, 65.
- Purse, B., Mellor, P., Rogers, D., Samuel, R., Mertens, P., & Baylis, M. (2005). Climate change and the recent emergence of bluetongue in Europe. Nature Reviews, Microbiology, 3(2).
- Rushton J, Lyons N. (2015). Economic impact of Bluetongue: a review of the effects on production. Veterinaria Italiana, 51(4), 401-406. doi: 10.12834/VetIt.646.3183.1.
- Saegerman, C., Berkvens, D., Mellor, P., (2008) Bluetongue Epidemiology in the European Union. Emerging Infectious Diseases. Vol. 14, No. 4. 539-544.
- The Center for Food Security & Public Health. (2023). Bluetongue. Recuperado de http://www.cfsph.iastate.edu/Factsheets/pdfs/bluetongue.pdf
- World Organization for Animal Health. (2021). Infección por el virus de la lengua azul. Código Sanitario para los Animales Terrestres, Capítulo 8.3. Recuperado de https://www.woah.org/fileadmin/Home/esp/Health_standards/tahc/current/chapitre_bluetongue.pdf
- World Organization for Animal Health. (2021). Lengua azul (infección por el virus de la lengua azul). Manual Terrestres de la OIE 2021, Capítulo 3.1.3. Recuperado de https://www.woah.org/fileadmin/Home/esp/Health_standards/tahm/3.01.03_Lengua%20azul.pdf
- World Organization for Animal Health. (2021). Bluetongue data sheet. Recuperado de https://www.woah.org/app/uploads/2021/03/bluetongue-2.pdf
- Wilson, A. J.; Mellor, P. S. (2009). Bluetongue in Europe: past, present and future. Philosophical Transactions of the Royal Society B: Biological Sciences, 364(1530), 2669–2681. doi:10.1098/rstb.2009.0091
DR. FRANCISCO JAVIER ORTEGO
Dr.Francisco Javier Ortego completed his PhD dissertation at Hospital Ramón y Cajal in Madrid under the supervision of Dr. Rafael Fernández Muñoz. He then pursued two postdoctoral studies for a total of 10 years, first at Yale University in the US and then at the National Center for Biotechnology of the CSIC in Madrid, where he held a reinstatement contract from the Ministry of Education and Science and an I3P contract from the Ministry of Science and Technology.
In February 2005, he joined the Animal Health Research Center (CISA-INIA/CSIC) as a Ramón y Cajal researcher and later became a Full Scientist of OPIs in 2008. Since then, he has been conducting research in the field of developing and optimizing universal vaccines against arboviruses relevant to animal health, focusing on the bluetongue and African horse sickness viruses.
In 2022, he participated in three projects, two as a Co-IP, which aimed to create and asses COVID-19 vaccines. Additionally, he served as Deputy of the National Agency for Evaluation and Prospecting (ANEP) from 2015 to 2017, and later as coordinator of the Livestock and Aquaculture (GYA) sub-area of the AEI from 2017.
We talked to Ortego to find out more about bluetongue and how best to prevent the disease.
BLUEVAC BTV
Line of inactivated vaccines to prevent and control the disease caused by the bluetongue virus (serotypes 1, 4, and 8) in sheep and cattle.
BLUEVAC-3
Inactivated vaccine to prevent and control the disease caused by the bluetongue virus serotype 3 in sheep and cattle.
BLUEVAC MULTISTRAIN
Line of inactivated vaccines to prevent and control the disease caused by several bluetongue virus serotypes (1+4, 4+8, and 1+8) in sheep and cattle.